The invention relates to a process for extending the life of a gas discharge vessel which is used as a radiation source, is permeable to radiation of a wavelength from 10 to 1,000 nm and has an activated cathode.
The invention also relates to a gas discharge vessel according to the process mentioned above.
Gas discharge vessels used as a radiation source, such as mercury vapor lamps, sodium vapor lamps or metal vapor lamps of other types, fluorescent tubes and the like, are as a rule equipped with so-called activated cathodes in order to improve their starting properties and their operating behavior. The activating substance applied to the cathode surface serves to reduce the work function of the metal compounds--preferably oxides--of the elements of the first three Groups of the Periodic Table (alkalis, alkaline earths and earths) are used in most cases. Above all, barium and its compounds are known from the literature for this purpose (for example Swiss Pat. No. 570,040).
The life of gas discharge vessels is largely determined by the processes taking place on the cathode surface. In the course of operation, both the activating substance and the cathode material vaporize or are atomized. The substances present, in most cases, in the elementary form, then precipitate on the inner walls of the discharge vessel and, in the course of time, reduce its permeability to the radiation to be emitted. For the usefulness of a vessel, however, its transparency is decisive. The particles precipitated on the inner wall--in particular the portions from the activating substance, which are present in the metallic form and which are relatively basic in nature and have a high affinity for oxygen--now react with the vessel material and change its chemical/physical properties in an unfavorable manner. The discharge vessels, predominantly made of glasses rich in quartz, become brown after a relatively short time and finally black and completely opaque ("blind"). This disadvantageous behavior in operation cannot be substantially improved by conventional measures such as adjusting the vessel temperatures, the gas filling, the operation of the cathode or the like.